This proposal seeks to study the effects of 24,25-dihydroxyvitamin in D3 as an adjunctive therapy for X-linked hypophosphatemic rickets. In this study patients on a standard therapy with Rocaltrol and phosphorus are randomized either to receive 24,25(OH)2D or placebo. The current focus is on the impact of this drug on secondary hyperparathyroidism and osteomalacia in adults with this disorder.